1. Field of the Invention
The present invention relates to a hydrostatic transmission (hereinafter, an “HST”), which is applicable to a transaxle apparatus.
2. Related Art
An integrated hydrostatic transaxle apparatus (hereinafter, “IHT”) comprises an HST, an axle, and a drive train interposed between the HST and the axle disposed together in a common housing which is filled therein with fluid so as to serve as a fluid sump. The HST includes a hydraulic pump and motor fluidly connected through a closed fluid circuit formed in a center section onto which at least one of the hydraulic pump and motor is attached. A fluid charge valve assembly may be attached to the center section so as to supply fluid from the fluid sump to the closed fluid circuit. The fluid charge valve assembly includes a check valve allowing only a fluid flow from the fluid sump to the closed circuit.
It is sometimes necessary to drain fluid in the closed circuit to the fluid sump or partly bypass the fluid into a short circuit. For example, if a proper amount of fluid is filled in the closed fluid circuit and a vehicle having the IHT is hauled, the axle of the IHT is necessarily rotated together with the hydraulic motor and the fluid in the closed circuit transmits the rotation of the hydraulic motor to the hydraulic pump. Thus, the hydraulic pump drivingly connected to a prime mover is rotated so as to transmit an opposite force to the prime mover. The drainage of fluid from the closed fluid circuit or the bypass circulation of part of fluid taken from the closed fluid circuit solves the problem.
There are some conventional IHTs that permit drainage of fluid from the closed circuit. In these conventional IHTs, the fluid draining means is separated from the fluid charge valve assembly.
Some of various IHTs are used for a vehicle having a vertical crankshaft engine, wherein the rotary axis of the hydraulic pump of the HST must be disposed vertically, i.e., in perpendicular to the horizontal axle. Thus, if the rotary axis of the hydraulic motor of the HST is disposed horizontally, the drive train between the HST and the axle can be simplified. However, this arrangement of hydraulic motor complicates the center section having the closed fluid circuit for fluid connection of the hydraulic pump and motor. Further, output means such as a motor shaft of the hydraulic motor must be extended horizontally, thereby expanding the IHT in the horizontal direction.
If the hydraulic motor is an axial piston type hydraulic motor, arranging a vertical motor shaft serving as a rotary axis of the hydraulic motor is available to reduce the horizontal size of the IHT for a vertical crankshaft engine and to simplify the center section of the HST. However, the vertical motor shaft projects from a cylinder block of the hydraulic motor through a swash plate abutting heads of pistons fitted in the cylinder block so as to expand the IHT vertically.
Usually, the swash plate of the hydraulic motor is a fixed swash plate and a retainer for the swash plate is fixed to the IHT housing. Preferably, the retainer is attached to the IHT housing without bolts or the like so as to facilitate assembly of the IHT and reduce the number of parts. Furthermore, a horizontal transmission shaft may be interposed between the vertical motor shaft and the axle (or a differential gearing). If the IHT housing is divisible into upper and lower halves, at least one of the upper and lower halves is usually provided with an integral vertically extending portion, or with a separate member fitted thereto, so as to support a bearing for the horizontal transmission shaft. However, if the swash plate retainer is also used for supporting a bearing for the transmission shaft, the housing (or the upper and lower housing halves) can be simplified or the number of members for supporting the bearing for the transmission shaft can be reduced.
Further, if an IHT is required to be vertically small, a vertically small structure for supporting an axle without a vertically expanded additional member is desired to be provided in a housing containing an HST, especially, between a wall portion of the housing and a center section of the HST.
Moreover, the vertical motor shaft is usually fitted at one end thereof into the center section. If the hydraulic motor is provided below the center section, an upper portion of the motor shaft is inserted into the center section. Thus, assembly of the hydraulic motor with the center section can be facilitated if the motor shaft is prevented from falling from the center section after the upper portion of the motor shaft is inserted into the center section and before the hydraulic motor is completely assembled with the center section. This may also be true of the hydraulic pump.